Evolution's Missing Link

Evolution's Missing Link

John Grotzinger has devoted his career to solving one of the most hotly debated mysteries in evolutionary biology-what happened hundreds of millions of years ago to cause the spontaneous burst of life that spawned the early ancestors of all animals on earth today. He returned to MIT in January 2002 after a year spent in the Middle East, where he collected fossil-containing rocks dating back to this so-called Cambrian Explosion. On the basis of those fossils, Grotzinger, a sedimentologist at the Institute’s Department of Earth, Atmospheric, and Planetary Sciences, believes he has found the answer.

According to the fossil record, life on earth arose 3.5 billion years ago in the form of tiny photosynthetic bacteria. For close to three billion years, the planet was devoid of anything larger than bacteria, plankton, and microscopic seaweed. Then suddenly, about 540 million years ago, in the dark depths of the ocean, a rich community of tiny animals sprang into being. From long spiny worms to five-eyed creatures with grasping hoses for mouths, they completely transformed the ocean floor about 10 million years ago-the blink of an eye in evolutionary time. These were the first representatives of virtually all major groups of animals on earth today and perhaps still other groups that disappeared long ago.

Scientists have come up with various explanations for the Cambrian Explosion. According to one theory, the appearance of predators might have triggered a period of rapid evolution: early prey animals, in their struggle to adapt, acquired defense mechanisms such as hard shells. Another possibility is that increases in atmospheric oxygen fueled the evolution of more complex body structures: oxygen is a requirement for skeletal development and growth. To clarify what actually did happen, Grotzinger went to Oman in January 2001 to look for fossils in rocks extracted from oil fields five kilometers below ground. “The benefit of these fossils is that they are very well preserved,” says Grotzinger.

When he brought his rock samples back to MIT for analysis, Grotzinger was astounded by what he saw. Together with two of his colleagues-geochronologist Sam Bowring and geobiologist Roger Summons-he reconstructed in three dimensions what the ocean basin would have looked like prior to the Cambrian period, the geological period during which the explosion of life occurred. Fossils contained in the rocks indicated that before the Cambrian period, the ocean reefs harbored early prototype animals with soft shells-Cloudina, a wormlike creature, and Namacalathus, a tiny animal shaped like a wine goblet. Once the Cambrian period began, however, these organisms simply disappeared.

By studying carbon isotopes in the rock, Grotzinger and Bowring determined that just before the Cambrian period began, a major environmental catastrophe occurred: the oceans suddenly stopped circulating. This event led to massive extinction of those early animals-possibly through a surge in such greenhouse gases as carbon dioxide and methane, released from the oceans at the moment the oceans started to circulate again. The demise of the early animals, Grotzinger maintains, cleared the playing field and allowed a new, more diverse, and better adapted group of animals to emerge.